The present invention relates generally to microwave reflectors and in particular to a microwave reflector incorporating a plurality of frequency selective or dichroic surfaces which selectively reflect and transmit different ones of a plurality of low, middle, and high frequency microwave signals and arranged to focus the low, middle, and high frequency microwave signals at physically displaced focal points.
Hyperbolic microwave reflectors are widely used elements of microwave communication systems. The microwave reflectors are frequently large, heavy, and costly. Size and weight are of particular importance when the microwave reflectors are components of a satellite-borne microwave communication system.
Dichroic surfaces that reflect signals in one frequency band and transmit signals in other frequency bands, have been used as subreflectors in conjunction with a primary microwave reflector for diplexing two widely separated frequency band microwave feeds. Using a dichroic surface, it is possible to separate two frequency bands, such as the S and Ku bands, for example, directing each to a separate feed. This allows microwave feed design to be optimized for each frequency band using a single primary reflector. The dichroic surface may, for example, reflect the Ku band waves and transmit the S band waves. A Ku band feed is placed at the point where the reflected Ku band waves are focused and the S band feed is placed at the location of where the S band waves are focused. Because the two focal points are at physically different locations, microwave feeds of the respective bands may be optimized. Such a diplexer is disclosed in the paper entitled "A Wide Scan Quasi-Optical Frequency Diplexer" by John J. Fratamico, Jr., et al., IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30 No. 1, January, 1982, and in the article "Design of a Dichroic Cassegrain Subreflector" by Vishwani D. Agrawal et al., IEEE Transactions on Antennas and Propagation, Vol. AP-27, No. 4, July, 1979.
Advanced communication satellites have been proposed for operation in three frequency bands. For example, the Advanced Tracking and Data Relay Satellite System will operate in the S, Ku, and Ka frequency bands. Other combinations of three frequency bands may also be used. It is desirable to have a microwave reflector which is able to separate the three frequency bands using a single primary reflector. Such a reflector will substantially reduce space and weight requirements in a satellite system.
It is therefore an objective of the present invention to provide a microwave reflector incorporating multiple dichroic surfaces capable of efficient operation in multiple frequency microwave communications systems. Another objective of the invention is to provide a microwave reflector in which dichroic surfaces are positioned between a microwave reflector and its focal point to selectively reflect and transmit different ones of a plurality of microwave signals, each in a different frequency band, and to direct the reflected and transmitted microwave signal to and from physically displaced focal points. Still another objective of the invention is to provide a microwave reflector capable of efficient triplex operation. Yet another objective of the invention is to provide a microwave reflector capable of triplex operation using a single parabolic primary reflector. Another objective o the invention is to provide a microwave reflector for us in multiple frequency satellite communication systems.